Various techniques have been developed for the fabrication of optical fiber preforms, including the double crucible method, the modified chemical vapor deposition method (MCVD), and the vapor phase axial deposition (VAD) method. The present invention is intended to provide an improved VAD method.
The VAD method can be carried out in various ways: for example, (1) a core soot is first formed and then converted into a transparent glass layer and, thereafter, it is covered with a silica tube, (2) a core soot is formed and converted into a transparent glass layer and, thereafter, a cladding is formed by the outside deposition method, or (3) core and cladding soots are simultaneously formed and then made transparent. The present invention is directed to, in particular, improvements of the third or simultaneous method as described above.
The simultaneous method is widely employed in the fabrication of soots for single mode fibers having a small core diameter as well as for graded-index fibers having a quadratic refractive index distribution. One difficulty encountered in performing the simultaneous method is that the cladding soot is breakable at the time of its formation or at the subsequent sintering stage of the preform including the core and cladding.
As a result of extensive investigations to overcome the above described problem, it has been found that the cladding soot is breakable for the reason that the temperature distribution from the inner boundary of the core soot to its outer periphery next to the cladding soot is not smooth as described hereinafter in detail. That is, if a spot where the core deposition temperature is lower is present inside the outer core periphery, the bulk density of the spot is lower than the surrounding area. This spot having a lower bulk density contracts when, after core deposition, it is heated to a temperature higher than the core deposition temperature by a flame of a burner for the formation of the cladding. The burners for use in the formation of the core and cladding are hereinafter referred to as core and cladding burners, respectively. Since the coefficient of contraction of the spot is higher than those spots where the deposition temperature and the bulk density are high, tensile force is exerted thereon, resulting in the formation of cracks. The formation of cracks at the sintering stage occurs for the same reason as described above.
Referring to FIG. 1, there is shown schematically a conventional method of forming core and cladding soots at the same time using core and cladding burners. In this case, a soot is formed using a core burner 4 and cladding burners 2 and 3. The temperature distribution of the soot surface formed by the conventional method of FIG. 1 is shown in FIG. 2. It can be seen from FIG. 2 that a spot where the deposition temperature is lower is present inside the core periphery and thus the temperature distribution from the inner boundary of the core soot to the outer periphery of the cladding soot is not smooth. In FIG. 2, T indicates a temperature and r indicates a distance from the center of the soot.